1. Field of the Invention
The present invention relates to an axis seal mechanism which is suitable for an axis of large turbo machines such as gas turbines, steam turbines, compressors, pumps, etc. The invention also relates to a turbine which generates motive power by converting the thermal energy of a fluid to kinetic rotational energy, and an axis seal mechanism therefor.
2. Prior Art
Generally, an axis seal mechanism is arranged around the axis of a gas turbine or a steam turbine in order to reduce the amount of gas leaking from the high pressure side of the axis to the low pressure side of the axis. FIG. 12 shows a leaf type seal 1 as one example of a conventional axis seal mechanism.
The leaf type seal 1 consists of planar plates 3 each of which has a predetermined width and which are arranged along the axis of the rotation axis 2.
The outer end of each planar plate 3 is welded to a casing 5 by a brazing part 4. The inner end of each planar plate 3 contacts the peripheral surface of the axis 2 with a predetermined pressure. As shown in FIGS. 12 and 13, the angle between each planar plate 3, the inner end of which contacts the axis 2, and the peripheral surface of the axis 2, which has a direction of rotation indicated by arrow d, is an acute angle.
The planar plates 3 are thus fixed to the casing and seal the peripheral surface of the axis 2 so as to separate the area around the axis 2 into a high pressure area and a low pressure area.
The casing 5 consists of a first baffle 7 at the high pressure side of the planar plates 3 and a second baffle 8 at the low pressure side of the planar plates 3, which are arranged in order to hold the planar plates between them and guide the action of the high pressure.
In the leaf seal 1 thus constructed, the inner end of each planar plate 3 floats away from the peripheral surface of the axis 2 due to the dynamic pressure caused by the rotation of the axis 2 so that the inner ends of the planar plates 3 do not contact the peripheral surface of the axis 2, which also prevents wear.
However, the above leaf seal 1 has the following problem.
The dynamic pressure caused by the axis 2 makes the planar plates 3 float away from the peripheral surface of the axis 2 so as to prevent wear and heating due to friction between the planar plates 3 and the axis 2. The first baffle 7 and the second baffle 8 are arranged so that the space between the first baffle 7 of the high pressure side and each of the planar plates 3 is the same as the space between the second baffle 8 at the low pressure side and each of the planar plates 3; however each planar plate 3 is subject to pressure from the high pressure side to the low pressure side which deforms it towards the radial center of the axis 2, which makes it difficult to maintain the floating state without contact between the inner end of the planar plate 3 and the axis 2.
In order to overcome the above problems, a mechanism wherein a flexible side leaf is arranged between each planar plate 3 and the first baffle 7 has been proposal. The outer periphery of the side leaf is welded to the first baffle 7 by spot welding.
The side leaf of the above leaf seal, when pressure is applied from the high pressure side, bends in the axial direction of the axis 2 and touches the side edge of the planar plates 3 and the space between the planar plates 3 and the first baffle 7 becomes smaller than the space between the planar plates 3 and the second baffle 8. Therefore gas flowing through the space between the axis 2 and the first baffle 7 flows from the inner ends of the planar plates 3 to the outer ends of the planar plates 3 and the planar plates 3 will float on the flowing gas.
However the above leaf seal with the side leaf has the following problem.
Because the side leaf of the above leaf seal is fixed to the first baffle 7, a bending force acts on the periphery of the side leaf as it bends towards the low pressure area. Because the periphery of the side leaf is welded to the first baffle by spot welding and the strength of the spot welding is relatively low, the side leaf may separate from the first baffle 7 due to the temporary bending force applied to the periphery of the side leaf, and the performance for sealing may become insufficient.
The present invention has been made in view of the above problems and seeks to reduce leakage of gas from the high pressure side to the low pressure side. The present invention also seeks to obtain an axis seal mechanism and a turbine which can maintain good sealing performance even at high differential pressures.
The axis seal mechanism of the present invention comprises a mechanism which prevents a flow of fluid along an axis through a cylindrical space between a stationary portion and the axis, and comprises casings which are supported inside of the stationary portion, a plurality of planar plates, which are mutually spaced in the circumferential direction of the axis, each of which is fixed to the casings and positioned so that an inner end of each planar plate makes an acute angle with a peripheral surface of the axis and contacts the peripheral surface of the axis over a predetermined length in an axial direction, a high pressure side plate which is arranged at a high pressure side of the planar plates and a low pressure side plate which is arranged at a low pressure side of the planar plates so as to hold the planar plates therebetween and a flexible plate which is arranged between the planar plates and the high pressure side plate and is flexible in the axial direction, wherein the flexible plate is fixed to each of the planar plates.
In the above axis seal mechanism, in a sectional view taken along a virtual plane perpendicular to the planar plate, the surface of each planar plate which faces the axis is designated as the bottom face, and the other surface of each planar plate is designated as the upper surface. In this sectional view taken along the virtual plane, the gas pressure which acts on the bottom surface of the planar plate is higher than the gas pressure which acts on the upper surface of the planar plate, therefore, the inner end of each planar plate floats and does not touch the axis.
Specifically, because a gas to which pressure is applied from a high pressure side tends to flow towards the low pressure side, and because the flexible plate is arranged between each planar plate and the high pressure side plate, and the space between each planar plate and the high pressure side plate is smaller than the space between each planar plate and the low pressure side plate, gas will flow from the space between the high pressure side plate and each planar plate in a diagonal direction along the upper and the bottom surfaces of the planar plates, and a low pressure area will extend to the outer end of each planar plate. The distribution of the gas pressure which acts on the upper and the bottom surfaces of each planar plate, when viewed in a section taken perpendicular to the width of each planar plate, forms a triangular shape, with the gas pressure tending to become lower from the inner end of each planar plate to the outer end of each planar plate. The distribution of the gas pressure on the upper surface is almost the same as the pattern of gas pressure on the bottom surface. However, each planar plate is inclined with respect to the peripheral surface of the axis, therefore, the pattern of gas pressure on the upper surface shifts with respect to the pattern of gas pressure on the bottom surface, and a gas pressure differential appears between one surface of the planar plate and the other surface of the planar plate at a given point on the surface.
The gas pressure Fb which acts on the bottom surface of the planar plate is higher than the gas pressure Fa which acts to the upper surface of the planar plate, therefore there is a force which makes each planar plate float away from the axis. Because the inner end of the planar plate is chamfered, there is no surface to receive the gas pressure near the inner end of the bottom surface of the planar plate, and only the upper surface of the planar plate receives the gas pressure near the inner end of the planar plate. However, the force which thus acts on the upper surface of the planar plate is cancelled by the pressure Fc due to the gas which flows between the inner end of the planar plate and the peripheral surface of the axis. Thus, the total force due to the gas pressure is (Fb+Fc) greater than Fa, and therefore each planar plate will bend so as to float away from the peripheral surface of the axis.
Because the flexible plate is fixed to the planar plates, the displacement in the flexible plate along the axis decreases compared to the case that the flexible plate is fixed to the high pressure plate, and a deformation in the outer periphery of the flexible plate decreases, so that the flexible plate does not become separated from the planar plates. As a result, the axis seal mechanism can maintain its sealing performance at a high differential pressure.
In the axis seal mechanism the outer peripheral surface of the flexible plate may be welded to each of said planar plates.
Because the flexible plate is strongly fixed to the planar plates, the flexible plate is prevented from separating from the planar plate as a result of a bending force due to a differential pressure between the high pressure side and the low pressure side.
In the axis seal mechanism the flexible plate may be inserted into a notched portion which is formed in each of said planar plates.
Because the flexible plate is fixed to the planar plates by insertion into the notched portion, it is not necessary to heat the planar plate and the flexible plate for assembly. This prevents the flexible plate and the planar plates thus assembled from being subject to heat deformation or damage, and degradation of the sealing performance is prevented.
The axis seal mechanism may also comprise a mechanism which prevents a flow of fluid along the axis through the cylindrical space between a static portion and the axis, comprising casings which are supported inside the static portion, a plurality of planar plates which are mutually spaced in the peripheral direction of said axis, the outer end of each being fixed to the casings, the inner end of each slidably contacting the peripheral surface of said axis at an acute angle over a predetermined length along said axis, a low pressure side plate, which is arranged in a low pressure side of the axis, and a high pressure side plate, which is arranged in a high pressure side of the axis, which hold the planar plates therebetween, a flexible plate which is arranged between the planar plates and said high pressure side plate and is flexible in an axial direction of said axis, wherein the flexible plate comprises a projecting portion which is held between the casing and the planar plates.
Because the flexible plate can be arranged between each of the planar plates and the high pressure side plate by providing the projecting portion of the flexible plate between the casings and the planar plates, when the outer ends of the planar plates are fixed to the casings, deformation and damage due to heating and forces applied during construction, and degradation of the sealing performance of the axis seal mechanism, are prevented.
In the axis seal mechanism, it is possible for the flexible plate to contact a side edge of each of the planar plates.
Because the flexible plate contacts the side edges of each of the planar plates and is supported by each planar plate, the force which acts on the outer periphery of the flexible plate becomes smaller. The flexible plate thus supported is prevented from separating from the planar plates and the sealing performance is maintained even at high pressures.
A turbine which converts thermal energy into kinetic rotational energy by blowing a high pressure and high temperature fluid flowing onto an active blade in a casing may comprise a turbine which consists of any one of the above axis seal mechanisms.
Because the turbine comprises the axis seal mechanism which reduces the amount of gas leakage even at high differential pressures, it is possible to minimize power loss due to leakage.